Distillation of hydrocarbon mixtures



Patented July 27, 1954 UNITED STATES PATENT OFFICE DISTILLATION OF HYDROCARBON MIXTURES Otto Redlich, Berkeley, and Charles M. Gable,

El Cerrito, Calif., assignors to Shell Development Company, Emeryvillc, Calif., a corporation of Delaware No Drawing. Application June 24, 1952, Serial No. 295,342

6 Claims. (Cl. 202-42) This invention relates to a process of distillavarious mixtures of vaporizable hydrocarbons by tion to separate components of h drocarbon mixdistillation in the presence of the perfluoro acid. "pg tures. It relates more particularly to the separa- Whereas perfluorocarbons are known to exhibit 5 tion by distillation in the presence of an exsolvent selectivity for the the more saturated traneous agent of mixtures of isomeric aromatic 5 (parafilnic) hydrocarbons, the replacement of all hydrocarbons. the hydrogen atoms in a carboxylic acid by The art of separatin various mixtures of 0rfluorine atoms gives an enhancement in the solganic compounds by distillation in the presence vent selectivity for the hydrocarbons which exor" an extraneous volatile substance which selechibit a more aromatic or a less saturated charactively changes the ratios of volatility of various in ter. The perfiuorocarboxylic acids are effective components of the mixture is well known and exagents for the distillation separation of mixtures tensive. The basis for the separation by azeo of hydrocarbons having difierent degrees of satutropic distillation and by extractive distillation is ration, such as the separation of naphthenes from the shift of the vapor pressure equilibrium caused paraflins, aromatics from naphthenes, aromatics by the extraneous agent, wherein the partial va- 7.5 from parafilns. Furthermore, they are effective por pressure of at least one component of the a ents for the distillation separation of mixtures mixture is changed sufficiently to permit its sepof closely boiling aromatic hydrocarbons, even aration by controlled fractional distillation. In isomeric aromatic hydrocarbons, particularly the separation of hydrocarbons of different demixtures of isomeric alkyl-aryl hydrocarbons grees of saturation, that is having difierent ra- :0 having alkyl groups of difierent sizes. tics of carbon to hydrogen atoms, by azeotropic The relative proportions of azeotroping agent distillation or by extractive distillation, there has and of hydrocarbon which are vaporized and reusually been efiected the separation 01 the rela moved as an overhead stream or product in an tively more saturated hydrocarbon as distillate azeotropic distillation for the separation of by product, together with azeotroping agent, leavdrocarbon mixture is dependent on the relative ing as undistilled bottoms relatively more unsatuvolatility of acid to hydrocarbon. The lower the rated hydrocarbons, such as aromatics, which relative volatility of the agent, the lower is the may or may not contain a portion of the exrelative proportion of such substance which is traneous substance in th case of azeotropic dis vaporized as compared with the other vaporized tillation, and which does contain selective solsubstance. It will be readily understood by those vent extraneous agent in the case of extractive familiar with the subject matter to which this distillation. invention pertains that the particular perfiuoro- Although the art is replete with information on carboxylic acid which will be selected for a parazeotropic and extractive distillation processes ticular separation by azeotropic distillation will. i for separating components of various rnxtures of depend in large measure on the boiling points of organic compounds, still there are available commercial mixtures of compounds of similar boiling points which are diilicult to separate effectively into their components. This is particularly true of mixtures of isomeric aromatic hydrocarbons. ,9

It is a principal object of the present invention to provide an improvement in the art of distillation for the separation of mixtures of hy the components of the mixture to be separated. Also, consideration must be given to mutual solu" bility relationships between the azeotroping agent and the hydrocarbon mixture as well as the component or components thereof which it is desired to be separated in the distillate. The azeotroping; agent should be substantially soluble in the hydroca bons A specific object of th invention is dmcarbon mixture i i p mp 0111.0 ns required to 1 f V 1 be used for the particular separation at the tem g to provide an imploved process for the sepaiain 1 tion of mixtures of isomeric aromatic hydrocarpel apure of the.dlstunatlon' .Recovely of the g bons by azeotropic distillation. Another specific tropmg agen? more read1ly efiected when t object is to provide an impmved extrastive mutual solubllities of azeotroping agent and dis tmatmn process for the separation of isomeric tillate hydrocarbon are not too great at ambient aromatic hydrocarbons. These objects will be atmospheric temperature or the temperature of mere f ll understocd and others will become the usual cooling water available for condenser parent from t detailed description of the service. On the other hand, other methods can be vention. utilized for recovery of the perfiuoro acid from the It has now been found that perfiuorocarboxylic hydro rb ns, uch s y xtr tin t hydroacids are efiective agents for the separation of carbon with a suitable solvent, for instance pen- 3 tane, or extracting the acid with a suitable solvent, such as water.

In the usual practice of the azeotropic distillation process of this invention, the overhead distillate may be cooled and condensed whereby the condensate separates into two liquid layers, the azeotroping agent predominating in the lower layer, which layer is returned as reflux to the azeotropie distillation zone, and an upper layer predominatin in the hydrocarbon. The azeotroping agent in the upper layer is readily recovered by a redistillation in a separate distillation zone whereby all of the agent is removed as an overhead azeotropic composition, which is recycled to the first azeotropic distillation zone, and a bottoms fraction free from the agent.

In accordance with a preferred embodiment of the invention, the azeotropic distillation is carried out in the presence of water, the mixture of perfiuoro acid and water requiring a smaller proportion of acid to give the same separating effect (alpha value), or, for the same proportion of acid (based on the hydrocarbon mixture), a greater separating effect being ob" tained. A further advantage in the use of a mixture of acid and water in the distillation zone is that the resulting presence of water in the overhead condensate enhances the separation of the acid and hydrocarbons into two separate phases, thereby simplifying the recovery of the perfiuoro acid. Still other advantages include the reduction of corrosion and an increased stability of the acid.

From the foregoing considerations and the results of experimental determinations, in general the perfluorocarboxylic acid should be selected for a good separation by azeotropic distillation so that its boiling point is within the temperature range of from about 35 C. below to about 35 C. above the 50% boiling point of the mixture to be separated, preferably being not more than about 25 C. above or below. Thus, for the separation of ethyl benzene from xylenes in a mixture thereof (B. P. about 137 C.), perfiuorobutyric acid (B. P. 122 C.) has been found to be an effective azeotroping agent. In the case of the lower boiling mixture, cyclohexane and benzene (B. P. 80 C.), trifluoroacetic acid (B. P. 72 C.) has been found to be particularly effective.

As will be understood, the azeotroping agent will be utilized in proportions sufiicient to form an azeotropic composition with a substantial proportion of the hydrocarbon component which it is desired to separate as distillate, and in general will be within the range of from about ten times to about one third of the weight of the hydrocarbon component which is to be separated as distillate. Also, in order to secure the most effective separation by the process, the distillation usually will be carried out in a plural-stage fractional distillation zone, as in a column provided with suitable trays, packing, or the like. When water is also utilized in the azeotropic distillation, it will be from about 0.5% up to about saturation in the hydrocarbon-acid mixture at the temperature of distillation, usually being from about 5% to about 20% by weight of the acid.

As will be understood, when a perfiuoro acid is to be used as extraneous agent in an extractive distillation, the agent will be selected so that its volatility is substantially lower than that of the mixture to be separated (B. P. preferably 35 C. higher) so that it functions essentially as perfluorobutyric acid.

a liquid phase selective solvent at the temperature of the distillation of the hydrocarbon mixture. For instance, perfluorobutyric acid is a suitable extractive distillation agent for the extractive distillation separation of a mixture of oyclohexane and benzene, with benzene being selectively separated in the distillation bottoms fraction. The benzene is readily separated from the separated extract phase by distillation and the solvent recycled to the extractive distillation zone.

By proper selection of the perfluoro acid, a portion of it can be removed in the distillate fraction and a portion in the bottoms or extract fraction, for instance the use of perfluoropropionic acid in the separation of cyclohexanebenzene mixtures or of ethyl benzene-xylenes mixtures.

In order to provide quantitative data for the very dii-licult separation of ethyl benzene from para-xylene, boiling points of 136.19 C. and l38.35 C., respectively, an approximately equimolal mixture of ethyl benzene and p-xylene was distilled in a single stage equilibrium still with The distillate split at room temperature in two phases with a separa tion factor 5 1.11 for ethyl benzene/p-xylene. The overall relative volatility distillate/bottoms for these components was a=l.19, compared with 0.0 1.06 for straight distillation. The detailed results are shown in Table 1. Similar determinations gave an alpha value of 1.22 for ethyl benzene/ni-xylene (ao=l.08).i

TABLE I Azeotropz'c distillation of ethyl benzene-pamazylene with perfluorobutyric acid Azeotropic temperatures and compositions of the binary systems of perfiuorobutyric acid with ethyl benzene, p-xylene and m-xylene are given in Table II.

TABLE II Azeotropes with perfluorobutyric acid Pcrfluorobutyric acid, percent wt.

Azeotropic Temp, C. at 760 mm.

Hydrocarbon The solubilities and critical solution temperatures of perfluorobutyric acid and ethyl benzene, p-xylene and m-Xylene are given in Table 131.

TABLE III mLiIydrol A d car onin 111. ci in 100 ml. Per- 100 m1. Hydrocarbon fiuoro- Hydro- Temperm butyric carbon at tum o O Acid at 25 C.

Ethyl benzcno 22 8 47. 5 p-Xylene 36 12 32. 5 m-Xylcne l4 32.5

The data in Table IV demonstrate the advantages to be gained by carrying out the azeotropic distillation in the presence of a mixture of the perfiuoro acid and water. Thus, comparison of the data in the first two columns shows that for the mixture of ethyl benzene-metal-xylene, for the same total solvent to hydrocarbon ratio (5.5) the same separation factor is obtained when about 20% Water (based on the acid) is present, as compared with the anhydrous acid. Similarly, with a solvent containing about 13% water and a slightly higher solvent ratio, the relative volatility was increased from 1.22 to 1.30.

TABLE IV Azeotropic distillation of ethyl ben2ene-metaxylene with, a mixture of perfluorolmtyric acid and water Test i A B 0 Percent wt. acid 84. 5 70. 6 75. 9 Percent wt. water 0.0 14.0 ll. 7 Acid/Hydrocarbon (weight ratio) 5. 5 4. 6 6.1 Total Solvent/Hydrocarbon (weight ratio) 5 5 5.5 7.1 Relative Volatility (Ethyl benzene/m- Xylenc) 1.22 l 21 1.30

Data for the azeotropic distillation of an approximately equimolal mixture of benzene and cyclohexane in the presence of trifluoroacetic acid are given in Table V.

TABLE V Azeotropic distillation of cyclohexane-beneene with trifluoroacetic acicl Relative Volatility (cyclohexane/benzene) =1.89.

The perfluorocarboxylic acids are suitable also for the separation of higher benzene derivatives, for instance, of mesitylene from methyl ethyl benzenes, or of cymene from butyl benzenes. In these separations the perfluorocarboxylic acids present the same special advantage as in the separation of m-xylene and ethyl benzene. In other hydrocarbon separations, such as the separation of aromatics and naphthenes, the perfluoro acids are comparable with other good azeotroping and extractive distillation agents.

We claim as our invention:

1. A process for the separation of a mixture of isomeric aromatic hydrocarbons having alkyl groups or different size by distilling the mixture in the presence of a substantial proportion of a perfiuorocarboxylic acid having a boiling point within about 25 C. of the 50% boiling point of the mixture and separating an overhead distillate portion which is enriched in one of the isomeric hydrocarbons and a bottoms portion which is enriched in at least another one of the iso meric aromatic hydrocarbons.

2. A process for the separation of a mixture of isomeric aromatic hydrocarbons having alkyl groups of different size by distilling the mixture in the presence of a substantial proportion of a perfluorocar'ooxylic acid having a boiling point within about 25 C. of the 50% boiling point of the mixture and of from about 5% to about 20 by weight of water, based on the amount of perfiuorocarboxylic acid, and separating an overhead distillate portion which is enriched in one of the isomeric hydrocarbons and a bottoms portion which is enriched in at least another one of the isomeric aromatic hydrocarbons.

3. A process for the separation of ethyl benzene from para-xylene in a mixture containing them by distilling the mixture in the presence of a substantial proportion of perfluorobutyric acid and separating an overhead distillate portion which is enriched in ethyl benzene and a bottoms portion which is enriched in para-xylene.

4. A process for the separation of ethyl benzene from a mixture of Cir-aromatic hydrocarbons containing ethyl benzene and at least one xylene by distilling the mixture in the presence of a substantial proportion of perfluorobutyric acid and separating an overhead distillate por tion which is enriched in ethyl benzene and a bottoms portion which is enriched in at least one xylene.

5. A process for the separation of ethyl benzene from a mixture of Cir-aromatic hydrocarbons containing ethyl benzene and at least one Xylene by distilling the mixture in the presence of a substantial proportion of a perfluorocarboxylic acid having a boiling point within about 25 C. of the normal 50% boiling point of the mixture and separating an overhead distillate portion which is enriched in ethyl benzene and a bottoms portion which is enriched in at least one xylene.

5. A process for the separation of ethyl benzene from a mixture of Ca-aromatic hydrocarbons containing ethyl benzene and at least one xylene by distilling the mixture in the presence of a substantial proportion of perfluorobutyric acid and from about 5% to about 20% by weight of water, based on the amount of perfiuorobutyric acid, and separating an overhead distillate portion which is enriched in ethyl benzene and a bottoms portion which is enriched in at least one xylene.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,279,194 Field Apr. 7, 1942 2,356,240 Hamlin Aug. 22, 1944 2,562,068 Souders July 24, 1951 2,614,965 Ruehlen Oct. 21, 1952 OTHER REFERENCES Journal of Research of the National Bureau of Standards, vol. 27, No. 1, pages 39-64 (July 1941). Article by Mair et a1. (pages 44, 49 and 56 only needed). 

1. A PROCESS FOR THE SEPARATION OF A MIXTURE OF ISOMERIC AROMATIC HYDROCARBONS HAVING ALKYL GROUPS OF DIFFERENT SIZE BY DISTILLING THE MIXTURE IN THE PRESENCE OF A SUBSTANTIAL PROPORTION OF A PERFLUOROCARBOXYLIC ACID HAVING A BOILING POINT WITHIN ABOUT 25* C. OF THE 50% BOILING POINT OF THE MIXTURE AND SEPARATING AN OVERHEAD DISTILLATE PORTION WHICH IS ENRICHED IN ONE OF THE ISOMERIC HYDROCARBONS AND A BOTTOMS PORTION WHICH IS ENRICHED IN AT LEAST ANOTHER ONE OF THE ISOMERIC AROMATIC HYDROCARBONS. 